See https://github.com/Maxner/BrightnessFix/blob/master/Makefile
ARCHS=armv7 armv7s
TARGET_IPHONEOS_DEPLOYMENT_VERSION = 6.0
TARGET_CC = xcrun -sdk iphoneos clang
TARGET_CXX = xcrun -sdk iphoneos clang++
TARGET_LD = xcrun -sdk iphoneos clang++
SHARED_CFLAGS = -fobjc-arc
include theos/makefiles/common.mk
TWEAK_NAME = BrightnessFix
BrightnessFix_FILES = Tweak.xm
BrightnessFix_FRAMEWORKS = UIKit
include $(THEOS_MAKE_PATH)/tweak.mk
SHARED_CFLAGS = -fobjc-arc
is what you are looking for.